Think your water bill’s high now? Wait about eight years, when District residents and some folks in the suburbs will see their average monthly bill exceed $100.

The hit to residential pocketbooks will come thanks to the region’s biggest all-but-unknown public works project. D.C. Water later this year will break ground on a $2.6 billion, 25-year project to build three tunnels deep underneath area waterways to store sewage so that it doesn’t end up in those waterways. And it’s being paid for mainly through water bills.

In other words, said George S. Hawkins, D.C. Water’s general manager, “It’s people who are paying this cost.”

Last year, D.C. Water started adding a new “impervious area charge” to customers’ bills to cover tunnel costs. What today is $3.45 a month for the average home will be more than $30 a month by 2019. The utility isn’t digging these tunnels for fun, mind you, but is doing so as part of a 2005 court agreement with the federal government and various community and environmental groups to stop dumping raw sewage into the river.

Some background: When it rains in the city, water drips off roofs and sidewalks and streets and parking lots and ends up down a drain. In the oldest portion of the city, there are no separate storm sewers, so the runoff gets mixed with sewage from sinks and toilets and other unsavory locales and sent to Blue Plains, the treatment plant at the southern tip of the city. When it rains, there’s more sewage than the plant can handle, and it overflows into the Anacostia and Potomac rivers and Rock Creek.

The tunnels offer a simple solution to the problem: Keep the runoff-and-sewage cocktail in a safe place until it can be treated. The problem is that tunnels are expensive and dangerous to build. They also do nothing to prevent storm-sewer runoff — full of dirt and grease and other nasty stuff — from ending up in rivers.

But what if there wasn’t so much rainwater dripping off roofs and sidewalks and streets and alleys and parking lots? That’s what Hawkins has in mind. Rather than spend billions on three tunnels, he’d like to build one tunnel and work on keeping water from going down the drain in the first place.

That includes building “green” roofs, which, instead of having hard shingles, are covered with dirt and vegetation that capture water, which then evaporates. There are green streets and alleys, too, which use plants and permeable materials that enable water to seep into the ground rather than run into storm drains.

Incidentally, Hawkins added, building green roofs, alleys and streets means good jobs for local residents. (Tunnel building requires skilled workers who travel from job to job.) And let’s not forget that more trees and plants make neighborhoods nicer places to live.

The ideas aren’t new but, in many ways, are newly feasible. Chris Weiss, who leads the D.C. Environmental Network, remembers proposing runoff prevention methods at a meeting about river issues early last decade. “I think we were pretty much laughed at,” he said. “But the technologies have come a long way. All of a sudden, this other stuff seems possible.”

And with Hawkins, a former Environmental Protection Agency lawyer, pressing them, they have a new champion. The EPA said in a statement that it “encourages” the inclusion of green infrastructure in consent decrees.

D.C. Water has started preparing a proposal, but while these are great arguments on paper, Hawkins’s challenge must sway the EPA, green groups and their lawyers, who aren’t all convinced that “low-impact development” will get the river as clean as fast as tunnels will.

David Baron, an Earthjustice lawyer who has represented environmental groups that signed the court agreement, said development methods have long been discussed as ways to keep rivers clean. But the tunnels are expected to eliminate 96 percent of the sewage flowing into the District’s waterways. It’s unclear whether green roofs, streets and alleys will do as effective a job.

“We don’t want to be in a position where we’re getting less in the way of raw sewage reduction,” Baron said. Also, Hawkins said, D.C. Water will ask for more time to make low-impact development work, which Baron calls a no-go. “The schedule for these tunnels is already pretty protracted,” he added. “It’s real easy for bureaucrats to lapse into the habit of delaying things whenever they change their approach. We’d want strong assurances that nothing’s going to get delayed.”

Hawkins said the overall benefit is worth a slightly longer wait. “The only reason we would propose this alternative is because the environmental outcomes might be better, not worse as a whole,” he said. “And, conceivably, to save out ratepayers some money.”

But when there’s a will, there’s a way, Baron said, pointing to the example of Nationals Park — located, incidentally, right next to one of the Anacostia’s biggest sewer-outflow locations.

“If they can do that in a couple of years,” he said, “what else they can do?”